CHAPTER 14- THE UNIVERSE
The universe includes all matter, energy and space.
Topics emphasized in this chapter are stars (including
the Sun), galaxies, the celestial sphere and cosmology.
Please do not confuse astronomy with astrology.
Astronomy is a true science with conclusions based on
fact and logic.
Astrology is a pseudoscience. Conclusions are
sometimes based on superstitions or beliefs that cannot
be scientifically verified. There are no known physical
quantities that could cause the effects claimed by
astrologers.
The Sun
The Sun is a star. It is a ball of plasma(a gas consisting
only of charged particles) at very high temperatures
held together by gravity. The high temperatures come
from energy released during fusion reactions within the
star.
The structure of the Sun can be explained by
considering layers from the outside to the center.
The photosphere is the bright, visible surface of the
Sun. The temperature here is about 6000 K and it is
composed of 75% hydrogen, 25% helium and very
small amounts of heavier elements.
On the surface of the Sun, huge patches of cooler
material sometimes appear called sunspots. Sunspots
appear in an 11 year cycle, increasing in number to a
maximum in the middle of the cycle then decreasing
until the end.
Flares are sudden explosive events originating on the
Sun's surface. They appear as streamers of solar
material being ejected into space around the Sun.
A prominence is an enormous filament of gas arching
over the surface of the Sun. Both ends are normally
connected to the Sun's surface.
The chromosphere is a layer of hotter gas(mostly
hydrogen) about 19000 km deep. Temperatures in the
chromosphere reach 50,000 K.
An artificial eclipse can be created using a coronagraph.
This is an instrument with an opaque disk that blocks
out the light from the photosphere. It is used to study
the chromosphere and corona.
The corona is a tenuous, white halo surrounding the
Sun that is only visible during a solar eclipse or with the
use of a coronagraph. Temperatures approach 1 million
K. The particles in the corona have enough energy to
leave the Sun. This outward flow of charged particles is
called the solar wind. The solar wind extends at least 50
AU out from the Sun's surface. This region is called the
heliosphere.
The interior of the Sun is composed of the fourth state
of matter called plasma. Complete atoms do not exist,
only a mixture of high speed nuclei and electrons that
move about independently as particles do in a gas.
The density of this plasma is 1.4 times that of water, so
it is not like any gas we are familiar with on the Earth.
All of the energy we receive on the Earth originates in
the Sun. The Sun's energy comes from the fusion of
hydrogen to helium in its core.
The fusion process that takes place in the Sun is thought
to occur in steps. The net reaction is:
4 1H1 → 2He4 + 2+1e0 + 2υ + 2γ + energy
In words, this reaction says that 4 protons combine to form one
helium 4 nucleus plus two positrons plus two neutrinos plus
two gamma rays and energy of motion of the products.
This net reaction is called the proton-proton chain and
results in some of the matter in the initial four protons
being converted into energy according to Einstein's
equation, E = mc2.
Notice that the Sun's mass is being used up at the rate
of 6.0 x 1011 kg of hydrogen every second. At this rate,
the Sun should last about another 5 billion years.
The Celestial Sphere
The celestial sphere is a huge, moving, imaginary sphere
surrounding the Earth. All of the objects visible in the
sky can be located by using coordinates associated with
the celestial sphere.
In the northern hemisphere, all the stars seem to
revolve around a point above the Earth's geographical
north pole called the north celestial pole. The closest
star to the north celestial pole is Polaris so it is known as
the North Star. Because the Earth's axis
precesses(wobbles), the identity of the North Star
changes over a period of 26,000 years.
The celestial equator is a circle located on the celestial
sphere directly above the Earth's equator.
The ecliptic is also a circle located on the celestial
sphere which traces the annual path of the Sun in the
sky. The ecliptic makes a 23.5° angle with the celestial
equator since that is the tilt of the Earth's axis.
The two locations where the ecliptic and celestial
equator intersect are called equinoxes since at this point
the Sun is directly over the Earth's equator and we have
12 hours of daylight and 12 hours of darkness. The
equinox in the spring, around March 21, is called the
vernal equinox and the one occurring in the fall, around
September 23, is called the autumnal equinox.
Two other important points on the ecliptic are the
summer solstice and the winter solstice. The summer
solstice occurs around June 21 and is the point where
the Sun is farthest north (highest) in the sky at midday.
In the northern hemisphere, daylight hours are longest
around this time.
The winter solstice occurs around December 21 and is
the point where the Sun is farthest south (lowest) in the
sky at midday. In the northern hemisphere daylight
hours are shortest around this time.
A half circle drawn from the north celestial pole,
through the vernal equinox and ending at the south
celestial pole is called the celestial prime meridian.
It is similar to the Earth's prime meridian which is used
as the starting point for longitude on the surface of the
Earth.
The position of a star or galaxy on the celestial sphere to
the east of the prime meridian is called right ascension.
It has a minimum of 0 hours and a maximum of 24
hours.
The position of a star or galaxy on the celestial sphere
above or below the celestial equator is called
declination. On the celestial equator, the declination
equals zero. Above the celestial equator, the maximum
value is + 90° at the celestial north pole. Below the
celestial equator, the maximum negative value is -90° at
the celestial South Pole.
The third celestial coordinate is distance. The straightline distance from the Earth to an object in the sky
completes the specification for the location of the object.
Distances are measure in astronomical units, light years
or parsecs. A light year is the distance light travels in
one year (9.5 x 1012 Km or 6 x 1012 miles).
A parsec is defined as the distance to a star when the
star shows a parallax of 1 second of arc. It is equal to
3.26 light years. The parallax equation relates distance
in parsecs to the parallax angle.
d = 1/p
p is the parallax angle in seconds.
Classifying Stars
Most stars are huge balls of plasma composed mainly of
hydrogen and some helium. Surface temperatures range from
1500 to 50,000 K and size can range from 0.08 to 100 solar
masses. These are the two main characteristics used to classify
stars.
Most stars are part of a binary system which consists of two
stars orbiting a center of gravity between them.
Constellations are groups of stars visible from the Earth that
form patterns and have been given names by early Babylonian
and Greek civilizations. Constellations have no physical
significance but they are useful in helping locate objects in the
night sky. 88 constellations have been identified.
Different constellations are visible in the night sky during
different times of the year because the Earth's orbit causes the
direction of the night sky to change from one season to
another. In the autumn night sky in the US, we see Pices and
Capricornus. In the winter we see Orion and Taurus. We see
Leo in the spring and Cygnus in the summer.
Constellations like Ursa Major with a declination of +50° or
more are visible all year.
Other groups of stars called asterisms are found within a
constellation or made up of stars from different constellations.
The Big Dipper is part of Ursa Major as the Little Dipper is
part of Ursa Minor.
Hipparchus, a Greek astronomer compiled the first star
catalogue around 129 B.C. He assigned them to six categories
of brightness called apparent magnitude. First magnitude stars
were about 100 times as bright as sixth magnitude stars.
Each magnitude difference of one corresponds to a brightness
factor of 2.51 which is 1001/5.
To compare the brightness of two stars, find the difference
between their apparent magnitudes and raise 2.51 to that
power. The star with the smaller apparent magnitude number
is brighter by a factor equal to the results.
ΔB = 2.51ΔM
Example
Sirius has an apparent magnitude of -1 and Polaris has a
magnitude of +2. Which star is brighter and by what factor?
Since Sirius has the smaller magnitude number, it is brighter.
The difference between the two is +2 - (-1) = 3
ΔB = 2.513 = 15.8
Sirius is 15.8 times brighter than Polaris.
The Sun has an apparent magnitude of -27 because it is so
close to us. The closest star other than the Sun is Proxima
Centauri, part of a three star system that is about 4.3 light
years away from us.
When comparing the actual brightness of stars is necessary to
introduce absolute magnitude. This is the brightness a star
would have if placed a distance of 10 parsecs from the Earth.
Absolute magnitude gives us better comparisons of actual sizes
and temperatures of stars.
The Hertzsprung-Russell diagram is a graph of star
characteristics placing absolute magnitudes on the Y axis and
temperatures on the X axis. On this diagram the temperature
increases to the left instead of the right.
Most stars on the H-R diagram get brighter as they get hotter.
This group of stars is called the main sequence. They form a
band going from upper left to lower right. Our Sun is a main
sequence star.
Stars found above the main sequence are brighter than their
corresponding temperature indicates so they are larger than
normal stars. These are called red giants and red supergiants.
Stars that are below the main sequence are not as bright as
main sequence stars of the same temperature so they must be
smaller. These are called white dwarfs.
The Life Cycle of A Star
The life cycle of a star starts with a cloud of dust and gas
known as a nebula. This may be a bright nebula which radiates
or reflects light or a dark nebula which blocks light from
material behind it. The Horse Head nebula in Orion is a dark
nebula.
As the material in the nebula begins to accrete, the
temperature begins to increase and a protostar is formed.
This process continues until fusion begins and at this point the
star is born and takes its place on the main sequence
depending on its temperature and brightness.
When the hydrogen is used up over the 10 billion or so years of
life, the star begins to expand to form a red giant.(A famous
red giant known as Betelgeuse is in Orion). Our Sun will
probably expand to engulf Mercury and Venus.
The core gets so hot that elements up through neon may be
created during fusion reactions. This is known as
nucleosynthesis.
During the red giant phase the star will become a variable star
changing brightness in a cyclic fashion.
Eventually it will blow off its outer layers to form a planetary
nebula and finally a white dwarf.
A white dwarf is about the size of the Earth but it is very
dense. One teaspoonful of matter from a white dwarf may
weigh five tons.
The Life Cycle of High-Mass Stars
The birth of a high mass star occurs the same way as the birth
of a low mass star. The difference is in the way they end their
lives as stars.
A high mass star can end its life as a supernova. A nova occurs
when a nuclear explosion occurs on the surface of a white
dwarf making it suddenly hundreds of times brighter that it
was.
A type I supernova occurs when a white dwarf accretes enough
matter to explode catastrophically. A type II supernova occurs
when the core of a massive red supergiant explodes.
The Crab Nebula is the result of a supernova.
Sometimes the remaining material will collapse into a neutron
star. A neuton star has no protons or electrons therefore no
atoms. Since no space is taken up by electrons orbiting nuclei,
the density of a neutron star is one billion tons per teaspoonful.
A neuton star as massive as the Sun would be much smaller
than the Earth.
Neutron stars spin very rapidly and emit radio waves in a
beam. When the Earth is in the path of this beam, astronomers
can detect a regularly occurring radio wave pulse. These stars
are then called pulsars.
If the remaining mass after a supernova explosion is large
enough, a black hole may be formed. In a black hole matter
has contracted to the point that it forms a singularity.
Anything(even light) coming within a limit called the event
horizon is trapped and becomes part of the black hole.
The distance from the center of the black hole to the event
horizon is called the Schwarzschild radius.
Black holes can only be detected by analyzing the
electromagnetic radiation coming from matter swirling around
them. Cygnus X-1 is thought to be a black hole taking matter
away from a class O super giant star near it. Cygnus X-1 is
smaller than the Earth, yet seven times as massive as the Sun.
About 100 million possible black holes have been detected.
Galaxies
A galaxy is a large collection of stars bound together by
gravity. We are in a spiral galaxy known as the Milky Way
Galaxy.
There are four classifications of galaxies based on shape,
elliptical, normal spiral, barred spiral, and irregular.
An elliptical galaxy has no curved arms and is shaped roughly
like a football.
A normal spiral galaxy has a nuclear bulge but also has
multiple arms that extend outward from the bulge in a curved
almost whirlpool like fashion.
A barred spiral galaxy has the shape of an elliptical galaxy
with a spiral arm extending from each end of the ellipse.
An irregular galaxy has no characteristic shape. The Large
Magellenic Cloud is an irregular galaxy.
The Milky Way Galaxy is surrounded by about 200 globular
clusters. A globular cluster is a group of about 10 to 50
thousand stars arranged in a spherical group. The discovery of
these led to the discovery of the center of our galaxy.
Our solar system is now thought to be about 28,000 light years
from the center of our galaxy in a spiral arm known as the
Orion arm.
Most galaxies are thought to have a super massive black hole
at the center. That would explain the high velocities of stars
orbiting near the center, the tremendous energy generated by
the core and the shape of most galaxies.
The most distant objects detected in the universe are called
quasars (quasi-stellar radio sources). The closest known
quasar is 600 million light years away and the farthest
currently detected is 13 billion light years away. The huge
amount of energy emitted by quasars is thought to come from
the fact that they are cores of galaxies that were forming when
the universe was young.
The amount of visible matter in the universe does not explain
the motions of the stars within galaxies or those of the galaxies
themselves. A type of matter called dark matter has been
introduced to help explain the gravitational behavior of stars.
Dark matter apparently is not seen with electromagnetic
radiation (light, radio waves, etc.) currently in use by
astronomers.
Cosmology
Edwin Hubble began looking a spectrum shifts of galaxies, a
way to tell if the galaxy is moving toward us or away from us.
He found that relatively close galaxies were doing both. Some
were approaching and some were receding.
When he examined galaxies farther out, he found they were all
receding. Red shift indicates that the frequencies emitted by
these galaxies had been apparently changed due to the
expansion of space towards the red end of the spectrum.
He concluded that the universe must be expanding.
Hubble's Law states the recessional velocity of a galaxy
depends directly on the distance from us to the galaxy.
The equation is:
v = Hd
Where v is the recessional velocity, d is the distance, and H is
the Hubble constant. The value of H is not exactly pinned
down yet. Once it is known, the age of the universe can be
calculated more accurately. Now it is known to be about 14
billion years old.
Cosmology is the branch of astronomy that studies the
structure and evolution of the universe.
The universe is thought to be expanding and even that the rate
of expansion may be increasing.
If the universe is expanding, there must have been a beginning
point and an accompanying expansion. This beginning point
and expansion is called the Big Bang. The idea is that
everything was compressed into one small, hot, dense state
where our current knowledge of the laws of physics did not
apply. Then a disturbance caused sudden expansion(the Big
Bang). Over the years, matter formed eventually into galaxies
and stars and all of the other objects in the universe.
The Big Bang Theory is supported by experimental evidence in
three major areas:
1. The cosmological red shift of the galaxies indicates space is
expanding.
2. The presence of the cosmic microwave background
radiation coming from all directions was predicted by the Big
Bang Theory. It is the remnant energy released during the
initial expansion.
3. The mass ratio of hydrogen to helium in the universe is 3 to
1 as predicted by the Big Bang theory.
A modification called the inflationary model is currently
considered the best theory. It differs from the standard model
in that it proposes the existence of many universes rather that
one. In this multiple universe theory we are only able to detect
events and objects in one which we consider to be "our"
universe.
P 406 Key Terms
P 406 – 409 Questions 1 – 51
P 409 Questions 2, 3, 5, 7, 9, 10, 11, 18, and 21